The general evolution in threat and weapon systems raises the need for improving the performance of weapons by virtue of a guiding system whilst seeking a minimum cost.
It is particularly advantageous, then, to reduce as far as possible the amount of sophisticated equipment loaded on board an expendable guided projectile (computer, homing head, inertial unit, proximity fuse etc.) by transferring the maximum number of functions to the firing station alone.
The guidance of a guided weapon launched by a conventional or electromagnetic gun and set into a rotation movement about its axis, may be performed with the aid of gas-generating lateral impellers, the operation of which is actuated when they are oriented in the desired direction. This requires knowing at any instant the roll angle of the projectile.
This function of angular measurement of the roll is generally ensured by an inertial unit (rate-gyro) loaded on board the projectile, which inertial unit is expensive and expendable. Furthermore, this inertial unit becomes difficult to design and to construct in the case of a projectile launched by an electromagnetic gun where the acceleration at the start may reach 200,000 g.
It is known how to measure the roll angle of a projectile with the aid of a catadioptric system equipped with a polarizer disposed at the rear of the projectile, with a light source illuminating the rear of the projectile and a light analyzer calculating the roll angle of the projectile from the polarisation direction of the light flux reflected by the rear of this projectile. However, this measurement has the drawback of exhibiting an ambiguity of .pi..